The strong growth in demand for portable consumer electronics is driving the need for high-capacity storage devices. Non-volatile semiconductor memory devices, such as flash memory storage cards, are becoming widely used to meet the ever-growing demands on digital information storage and exchange. Their portability, versatility and rugged design, along with their high reliability and large capacity, have made such memory devices ideal for use in a wide variety of electronic devices, including for example digital cameras, digital music players, video game consoles, PDAs and cellular telephones.
While many varied packaging configurations are known, flash memory storage cards may in general be fabricated as system-in-a-package (SiP) or multichip modules (MCM), where a plurality of die are mounted and interconnected on a small footprint substrate. The substrate may in general include a rigid, dielectric base having a conductive layer etched on one or both sides. Electrical connections are formed between the die and the conductive layer(s), and the conductive layer(s) provide an electric lead structure for connection of the die to a host device. Once electrical connections between the die and substrate are made, the assembly is then typically encased in a molding compound which provides a protective package.
In order to most efficiently use package footprint, it is known to stack semiconductor die on top of each other. In order to provide access to bond pads on the semiconductor die, the die are stacked, either completely overlapping each other with a spacer layer in between adjacent die, or with a stepped offset. In a stepped offset configuration, a die is stacked on top of another die so that the die bond pads of the lower die are left exposed. This allows wire bonds to be formed on the die bond pads of the semiconductor die at each level.
As semiconductor die become thinner, and in order to increase memory capacity in semiconductor packages, the number of die stacked within a semiconductor package continues to increase. However, this can make for long bonds wire from the upper die down to the substrate. Long wire bonds are easily damaged or electrically shorted to other wire bonds, and also have higher signal to noise ratio than shorter bond wires. Moreover, larger numbers of semiconductor die in a package can adversely affect yields.